1. Field of the Invention
The present invention relates to a method and apparatus for continuously detecting the level of glucose in a person, and for automatically delivering glucose or glucagon, and insulin to the person to maintain the glucose level in a desired range. The invention also relates to a method and apparatus for regulating other conditions or constituent levels in a person, tissue, or animal by continuously sensing the condition or constituent levels and delivering an agent and counteragent for maintaining the condition or constituent levels in a desired range.
2. Description of the Related Art
One of the more advanced methods of drug delivery is one in which the amount of a drug delivered is controlled by a sensor. The sensor reads, for example, the concentration of a constituent in the blood or tissue, or the magnitude of the results of a previous drug delivery, and alters the quantity or rate of delivery of a drug for altering the detected constituent concentration. For example, in a system for insulin delivery, the sensor can read the blood glucose level and vary the insulin delivery amount or rate accordingly.
At present there are very few sensors available which can function in this manner. One such sensor is disclosed in commonly owned U.S. Pat. Nos. 5,101,814 and 5,190,041, and U.S. patent application Ser. No. 08/077,893, the contents of which are incorporated herein by reference. This sensor comprises a mass of living cells which are sensitive to a particular constituent, preferably glucose. The cells are encapsulated in a biocompatible semi-permeable membrane and generate a detectable electrical signal which varies with the concentration of the constituent in the medium that surrounds the cells or capsule.
Most existing feedback controlled drug delivery systems suffer from the inability to lower the drug level, or its efficacy, once delivered. Thus, any overdose cannot be counteracted and one has to wait until natural metabolism and excretion lower the level of the agent administered.
There are however cases where a counteragent is available. For example, in the case of insulin hormone delivery, if an overdose is sensed by the sensor in the form of a too low glucose level, the situation can be corrected by delivery of glucose, or the hormone glucagon, etc.
It is therefore an object of the present invention to improve sensor controlled delivery systems by providing the capability to automatically deliver either a chemical agent or counteragent based on continuous sensor readings to maintain the level of a constituent or condition, for example blood pressure, within a desired range.